1. Field of the Invention
The present invention relates generally to a fuel injection control system for an internal combustion engine, such as an automotive internal combustion engine. More specifically, the invention relates to a fuel injection control system which can precisely control the fuel metering amount depending upon variation of an engine load. Further particularly, the invention is directed to an improvement in and relating to a so-called sequential fuel injection system which is designed for injecting fuel for each engine cylinder at before and/or during piston induction stroke and thus is known for precise engine load dependency in adjustment of the fuel metering amount.
2. Description of the Background Art
Recently, higher fuel economy, higher engine response, higher engine performance and achievement of anti-polution are required for an automotive internal combustion engine. In response to such demand for the automotive engine, various control systems for the automotive engine and improvements thereof have been developed. For example, the Japanese patent first (unexamined) publications (Tokkai) Nos. Showa 60-195347, Showa 60-233352, Showa 60-233353, Showa 59-211731 and Showa 59-29733 have been proposed for precise fuel injection control for the automotive internal combustion engines.
In the proposed fuel injection control systems, fuel injection start timing and fuel injection amount are derived on the basis of an engine revolution speed and and an engine load condition. Fuel enrichment by increasing fuel metering amount is performed during engine warm-up condition, during engine accelerating condition and so forth. The fuel injection start timing is adjusted so that the injected fuel may form air/fuel mixture with intake air to be introduced into the engine combustion chamber during induction stroke. Consequently, during acceleration enrighment, the fuel injection start timing is advanced so that the increased amount of fuel can be introduced into the combustion chamber during the engine induction stroke.
Generally, the fuel injection start timing is set in synchronism with the engine revolution cycle. The fuel injection to be performed in synchronism with the engine revolution cycle will be hereafter referred to as "synchroneous injection". In addition, during acceleration enrichment, additional fuel injection can be performed irrespective of the engine revolution cycle for increasing fuel delivery amount. Such fuel injection which is not synchroneous to the engine revolution cycle will be hereafter referred to as "asynchroneous injection". In the most of the fuel injection control, asynchroneous injection is taken place in response to occurrence of the acceleration enrichment demand which is caused by increasing of an open angle of a throttle valve and/or increasing of an intake air flow rate (engine load).
The sequntial fuel injection system is known by good response characteristics to variation of the engine load since the fuel injection amount can be determined utilizing most recent engine driving parameters, such as engine speed, engine load and so forth. In view of precise engine load dependent fuel delivery amount control, it is preferable to determined the fuel injection amount and fuel injection start timing as late as possible. On the other hand, in order to form uniform mixture rate of air/fuel mixture which is essential to obtain good frame propagation characteristics during combustion, it is essential to sufficiently atomize the injected fuel. In order to achieve satisfactory atomization of the injected fuel, it is preferable to set the fuel injection timing sufficiently before staring of open of an intake valve which timing is the beginning of the induction stroke. These two requirements about the fuel injection timing conflict to each other.
Therefore, in the prior proposed fuel injection control systems, it was difficult or even impossible to full satisfy both requirements. Therefore, the prior proposed fuel injection systems could not optimize the engine response to variation of the engine load.
In order to solve such difficulty in the prior proposed sequential fuel injection controls, the Japanese utility model first publication (Jikkai) Showa No. 61-76143 proposes an improvement for the sequential fuel injection by performing fuel injection twice in each combustion cycle. Namely, in this proposal, first fuel injection is performed at a timing sufficiently advanced from the beginning of the engine induction stroke for better fuel atomization characteristics and second fuel injection is performed at a timing delayed as late as possible for compensating fuel amount to be delivered in the engine cylinder relative to engine load variation.
This may substantially improves fuel delivery characteristics and significantly improves engine response to variation of the engine load.
This prior proposed system essentially divides the necessary fuel injection amount for the aforementioned first and second fuel injection timings so that total amount of the fuel injected at both of the first and second fuel injection timings may corresponds to the required fuel amount. However, when the substantial change in the engine load occurs after setting of the fuel injection amount for the first injection, which may require acceleration enrichment, only half amount of fuel for acceleration enrichment may be increased for the second fuel injection. In addition, at the initial stage of engine acceleration, an air flow rate measuring the intake air flow rate delays in detection of the actually introduced air. Therefore, an air flow rate indicative signal to be used at the initial stage engine acceleration, will indicate smaller engine load indicative value than that actually introduced. Therefore, at the initial stage of acceleration, the air/fuel mixture to be supplied to the engine cylinder becomes lean. In the worst case, such lean mixture may leads mis-firing for degrading engine acceleration characteristics.